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UC2843AQ: About Recommended RT and CT for Oscillator Frequency

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Guru 21045 points
Part Number: UC2843AQ

Hi Team,

 

I have three questions.

 

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[Q1]

We would like to know the meaning of “RT > 5k” of equation.

If we select less than 5kohm, what kind of problem is there?

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[Q2]

I understand that Recommended range of RT is from 300ohm to 10kohm and CT is from 470pF to 4.7nF depended on Figure6 Graphs.

Is my understanding correct?

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[Q3]

We would like to know Recommended combination of 400kHz and meaning of “Maximum Duty Cycle vs Timing Resistor”.

 

If setting frequency is 400kHz and Duty is 90%, Recommended combination is RT is 7.5kohm and CT is 560pF depended on Equation.

If setting frequency is 400kHz and Duty is 80%, Recommended combination is RT is 3kohm and CT is 1400pF depended on Equation.

 

Is my understanding correct?

---------

 

Regards,

Hide

  • 0
    TI__Genius 13660 points

    Hi Hide,

    1/. Using a value of 5 kΩ or greater for RRT maintains a favorable ratio between the internal impedance and the external oscillator set resistor and results in minimal change in frequency over temperature. Using a value of less the recommended minimum value may result in frequency drift over temperature, part tolerances, or process variations.

    2/. The recommended range of timing resistor values is between 5 kΩ and 100 kΩ.

    3/. The recommended range of timing capacitor values is between 1 nF and 100 nF.   

    We can limit the maximum duty cycle by regulating the dead time.  The maximum duty cycle limit can be set by the ratio of the external oscillator charging resistor RT and the internal oscillator discharge transistor on-resistance. But you should ensure that RT and CT are in the recommended range when you design the limited maximum duty cycle. If the RT and CT exceed the recommended range while limiting the required maximum duty cycle, it can not reliably be set in this manner. To control a lager limited range of maximum duty cycle, consider using the UCCx807. 

    Regards,

    Teng


     

  • 0 in reply to Teng Feng
    Guru 21045 points

    Hi Teng-san,

    Thank you for your prompt reply and detail information.

    Regards,

    Hide